.minhaClass {
    background: url('/images/NovoPortal.jpg') no-repeat;
    -webkit-background-size: cover;
    -moz-background-size: cover;
    -o-background-size: cover;
    background-size: cover;
}

@media only screen and (max-width:960px) {
    .minhaClass {
        background: url('/images/NovoPortal-mobile.jpg') no-repeat;
        -webkit-background-size: cover;
        -moz-background-size: cover;
        -o-background-size: cover;
        background-size: cover;
    }
}